This relates generally to image sensors, and more specifically, to image sensors containing pixels with adjustable body bias voltage.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. Conventional image sensors are fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology. The image sensors may include an array of image sensor pixels each of which includes a photodiode and other operational circuitry such as transistors formed in the substrate.
Capturing images using a CMOS image sensor often involves using an electronic rolling shutter (ERS) algorithm to successively reset, integrate, and read out single rows of image pixels on the image sensor. In the traditional ERS algorithm, row reset and readout are typically performed for a single row at a given time. Row reset refers to an operation which prepares a pixel for light capture by resetting a charge storage node to a given voltage. Row readout refers to an operation on image pixels that have been exposed to light for a desired duration of time, which involves sampling the pixel columns of a given row and converting a value related to the amount of charge stored by the pixel during exposure to a digital signal.
Image sensors typically include a photodiode having a pinning-voltage which is a design parameter set by the doping levels of the photodiode. During normal operation, a photodiode node is first reset to the pinning-voltage using transistor circuitry. Then photons are allowed to enter the photodiode region for a pre-defined amount of time. The photons are converted to electrons inside the photodiode area, and these electrons reduce the reset pinning-voltage. In this process, the total charge stored, Q, is commonly referred to as the saturation full well (SFW) and depends on the well capacity of the photodiode. When it is time to read out the stored signal, the stored charge Q at the photodiode node is transferred to a floating diffusion node through additional transistor circuitry. Pixel design should maximize the amount of charge Q that can be transferred from the photodiode to the floating diffusion node. If not, the charge spill back manifests as a loss to image quality. Alternatively, image sensors are often operated in global shutter (GS) mode. In this mode, an additional diode and an additional transfer gate are typically formed at the front surface of the substrate adjacent to the photodiode. In GS, all photo-diode accumulated charge is transferred to duplicate diodes in one global pulse, and the duplicate diodes are read row by row in ERS mode.
There are many sources of noise that may degrade the captured signal Q. Dark-current refers to electrons generated and captured by a photodiode from non-photon sources. Dark-current can originate from many sources including: Si defects due to implant & plasma damage, metallic contaminants in photodiode volume, avalanche and/or Zener high field electro-hole pair generation, SRH electron-hole pair generation, trap related band-to-band-tunneling (BTBT), transfer gate induced BTBT on both photodiode and floating diffusion sides, and many others. In order to achieve high image quality, dark-current must be reduced. In GS pixels, the duplicated diodes must preserve the charge transferred during the entire read-out time. Any disturbance in charge is extra noise in the form of Global-Shutter-Efficiency (GSE). It is desirable to get very high GSE.
It would therefore be desirable to be able to achieve high photodiode well capacity and minimal noise without sacrificing image quality.
CMOS image sensors are used heavily in the mobile industry in cell phone and PDA applications. These products require low power consumption to increase battery life. Pixel operating voltage does not scale to lower voltages easily due to pinning and charge transfer limitations. It is further desirable to reduce maximum pixel operating voltage to reduce power, while not degrading image quality.